1. Field of the Invention
This invention relates to an image data processing apparatus and an image data processing method, and more particularly to an apparatus and method which are applied effectively to an image processing apparatus, such as a copier, a printer, or an image scanner.
2. Description of the Related Art
Image information is generally compressed and stored for subsequent use because of its large volume. In the field of color image data processing technology, the Joint Photographic Experts Group (JPEG) method has been particularly popularized. Not only in the JPEG method but also in an irreversible color image data compression method, image data is generally converted into luminance and color-difference signals using human visual characteristics and the resolution of color difference is converted into a lower resolution than that of luminance, thereby compressing the data.
The decoded image is generally used as an image obtained by up-sampling the color difference and making the resolution of the color difference signal and that of luminance signal the same. The reason is that the input image size is made the same as the output image size to enable the output side to view the same image as that of the input.
On the output side, the process of outputting an image N times (as large or small as) the input image and the process of decoding a partial region are also included in a form of use of compressed data on the decoding side.
As described in reference 1 (Jpn. Pat. Appln. KOKAI Publication No. 2000-175061), a decoding method has been devised which uses the relationship between the resolution of luminance and that of color difference in returning the luminance and color difference signals to the RGB signals to reduce the number of data conversions. On the other hand, as for compression, a method has been devised which does not simply compress an image, but divides an image into a character area and a non-character area and uses different compression methods for the respective areas as described in reference 2 (Jpn. Pat. Appln. KOKAI Publication No. 2003-338934).